To use an auto air conditioning manifold gauge set, close the low and high valves on the set and connect the hoses to the low and high fittings on the air conditioner unit. Turn on the engine. Set the air conditioning to the maximum setting and run it until the car becomes cool. Reopen the manifold gauge set's valves and note the pressure readings that appear on its dials. Compare these readings to the specifications for the vehicle.
High pressure readings on the auto air conditioning manifold gauge set's high valves are often the result of excess refrigerant. Air in the air conditioner's hoses or restricted airflow through the condenser could also be the culprit. High valve readings on the low end indicate a low refrigerant level or malfunctioning compressor. Low valve pressures that are higher than expected may be due to a bad compressor or refrigerant that has been overcharged. Lower valve pressures typically mean the refrigerant level is too low or the condenser's air flow is blocked.
Auto air conditioning manifold gauge sets are used to check the pressure in air conditioning lines. They help identify if a leak is present. Air conditioning manifold gauge sets can also drain the unit of all refrigerant or recharge it fully.